Manganese stabilizing protein (MSP) is essential for the redox activity of Mn atoms at the active site of photosystem II, which oxidizes water to oxygen. Site-directed mutants of spinach MSP are created in an E. coli overexpression system, purified and tested for activity by replacing native MSP with recombinant protein. Sites for mutagenesis are determined by sequence comparisons among a number of species; highly conserved amino acids are targeted for mutagenesis. The purpose of these experiments is to test the hypothesis that this protein facilitates Mn redox activity either by providing ligands directly to Mn atoms at the active site of water oxidation, or by controlling the structure of other proteins that provide metal ligands.